I would be worried about the brakes and suspension getting corrosion and then not working properly if the motorhome is not moved at least monthly.  I know I had that problem with the brakes on our Monaco Knight which had hydraulic disc brakes.  Maybe this isn't a problem with air brakes, but you might want to ask a good motorhome tech. 

My preference for long term sitting in one place would be a 5th wheel trailer.  There is no engine or transmission to worry about and the brakes and suspension are much less complex than on a large Class A motorhome.  If fact, after owning 2 Class C and 1 Class A motorhome, we much prefer the space and layout of a 5th wheel for fulltime living.  However, I say that without yet having owned a 5th wheel trailer!  We hope to correct that situation someday!

-- Edited by wmalefyt at 12:41, 2008-10-15

If you want to camp in those public campgrouns a lot then a 42' motorhome is going to limit you and I can say this with a 40' motorhome.  The normal wisdom is about 33' is the maximum you can have and keep your choices open.  Even at 35' there are places that is too large.  These numbers come from multiple discussions on various forums over the years.  Many forest service and such campgrounds were built for 30' or less but 32' to 33' RVs can squeeze in. 
I know lots of people who let their motorhome sit for a few months every winter while they are in a snowbird campground and don't have any problems.   Can't say about summer since most people I know are on the move in summer.  I do know that being close to salt water can be bad.  A gas generator needs to be exercised monthly due to varnishing, LP and diesel can sit longer.  Covering your tires is a good idea when sitting long and RV supply places like Camping World have the covers.
Motorhomes, especially big diesel pushers, have more electric need than fifth wheels, at least on the 12 volt (battery) side.  You will need to run the generator more than your buddy with the trailer.  I know we do.

I think you are trying to have it all and are going to have to compromise somewhere.  I also realize that this is a joint decision and the real problem might be getting you both to agree on things. What RV you buy and how you want to camp are related.  If you want big, you will have to compromise on where you can stay and how much fuel you will use.  If you want to blacktop boondock (Wal-Mart, etc.) you better make sure you can do so with the slides in.  If you want to take it all with you then you have to have a big RV with lots of carrying capacity.  Are you going to want 4WD since you like the national forests?  Lots more questions can be asked, but really think this through and ask yourselves the hard questions.

bjoyce has it, as usual....

With either rig, it's not going to be ground clearance, but height. Some rigs top 13ft and those beautiful tree lined small town Main Streets are a huge problem. Trees are almost always blocking the right lane at that altitude. "Primitive" CGs - unless a wide open field - are seldom able to handle the big boys (turn radius, ground obstructions, trees, width, ruts) a moho being a significant disadvantage as they are less maneuverable than a 5er (mohos don't bend in the middle!). Weight - especially on dirt fields - is a potential sticking point and again mohos are heavier especially per axle - hitting 20,000lbs/axle.

The trade off will be take it all or compromise like Bill says. Even some Walmarts can be a problem due to curbing designs. Trip planning goes up as the square or even cube of increasing length. Those height and weight issues requiring you to know exactly what is ahead before you head off. You are not likely to be able to just stop off and visit the largest ball of twine or pick up a Frosty whenever you desire while driving through to your destination. U turns with 42/45ers under tow is, well let me just say, interesting. You also just can't "park" these babies totalng some 65ft in length and to practice "stealth" camping is, well, just a tad obvious. And then you happen upon that 15ton bridge or road construction area and have to figure out how to turn that 22ton 65foot rig around on a 2 lane secondary road! Now that is fun! Hence, the larger rigs push one towards "destination" based travelling as opposed to those who prefer that half the adventure is the "journey" itself.

With that said, my choice was carrying capacity and I configured it for boondocking so I deal with all the mobility and parking issues above. It does add a lot of planning and a lot of Google Earth close up previewing of all locations!  But once I get there, boondocking or otherwise, I do have everything - including more batteries than Exide - to really enjoy where I am. It is a choice and a comprimise. Until I can afford a Uni-Cat, I'll stick with the big boy moho.

Terry & Jo,
Where in Oklahoma City are you planning to 'live' in your rig?  We are also from Oklahoma (Edmond).  We sold the house and almost everything in it and hit the road full-time 4 years ago.  We have a 40ft. motorhome and we tow a Jeep Liberty.  Our height is 13'1" and we can relate to all the issues RVDude referred to.  We've had those 'Oh, my God" moments when the dreaded "Low Clearance" words came out of nowhere. 
We've been told by former neighbors that Oklahoma City has held up very well through the housing crisis.  I read somewhere it was one of the most 'recession proof' cities in the nation.  Good Luck with the sale of the house and we wish you the best in your selection of an RV for your future home.